Imaging with near field microscopy is discussed in A. Lewis, M. Isaacson, A. Harootunian and E. Muray, Ultramicroscopy 13 227 (1984); A. Harootunian, E. Betzig, M. Isaacson and A. Lewis, Appl. Phys. Lett. 49, 674 (1986); R.C. Reddick, R.J. Warmack and T.L. Ferrell, Phys. Rev. B, 39, 767 (1989); D.W. Pohl, W. Denk and M. Lanz, Appl. Phys. Lett., 44, 651 (1984); U. Durig, D. Pohl and F. Rohner, J. Appl. Phys. 59, 3318 (1986); U. Ch. Fischer, J. Vac. Sci. Technol. B3, 366 (1985); S. Okazaki, H. Sasatani, H. Hatano, T. Hayashi and T. Nagamura, Mikrochim. Acta (Wien), III 87 (1988); and D. Courjan, K. Sarayeddine, M. Spayer, Opt. Commun. 71, 23 (1989). Molecular exciton microscopy is discussed in K. Lieberman, S. Haroush, A. Lewis and R. Kopelman, Science, 247 59 (1990).
For the past several years there has been an increasing interest in developing a lensless methodology of light microscopy and lithography in which a small beam of light is scanned over a surface and either measures the optical properties of a surface as a function of the position of this beam of light, or patterns a surface, with resolutions that depend on the dimension of the point of light rather than the wavelength of the light. There have been basically two problems with this methodology of super-resolution light microscopy and lithography. The first problem involved trying to produce a point of light that was much smaller than a wavelength and still had enough intensity to produce images of low contrast objects by serially accumulating point by point information on the surface of an object. The second problem was a problem of feedback control, for it was difficult in a general fashion to control the distance between the point of light and the surface to be imaged. This distance is critical to the resolution that can be achieved in this lensless method of microscopy. This arises from the fact that in order to produce such serial images at a resolution that is much smaller than the wavelength of the light, the distance between the point of light and the surface has to be accurately controlled. In fact, the point source of light has to be within a distance known as the near-field, which is smaller than the wavelength of the light being used for the imaging. In other words, there has to be a method of feedback that maintains the point of light at a set distance from the surface.
In terms of the methodology of producing the point source of light, there were several stages in the development that have led to the ability to produce subwavelength spots of light. The history of these methodologies can be described in the following way. First, methodologies were found to produce small holes in a flat metal plate and light had to be passed through these small holes. It was seen from these experiments that light could be passed through holes that were smaller than 1/20 the size of the wavelength of the light. However, this methodology of producing spots of light smaller than the wavelength was not effective in producing a microscope since it was difficult to place such a point source in a flat plate relative to the surface in the near-field. In addition to this approach there was a methodology that allowed a point of subwavelength light to be generated at the tip of a long rod. There were generally two methods to this approach for producing of subwavelength spots of light. The first of these used a single crystal quartz etched to a tip and then coated with metal. Subsequently a small aperture was produced at the tip, which aperture appeared to be less than the wavelength of visible light (500 nm). Within this general method can also be categorized the use of solid optical fibers in which are created a subwavelength tip. All of these methods can be classified as prior knowledge based on the work of Synge (E.H. Synge, Phil. Mag. 6, 356 (1928).
An alternate approach was to heat and pull a hollow glass capillary into a pipette and produce an aperture at the tip which was smaller than the wavelength of light. This successfully allowed the production of small tips that could, under the right conditions, be as small as 7.5 nm. The problem with both of these approaches was that as the aperture got smaller, the throughput of the light became very low. This was also the problem with approaches using solid glass tapered structures. Nonetheless, the methodology of pipette formation allowed an alternate procedure to increase the throughput of the light passing through the hole. This approach involved the growing of small crystals that allowed the photons of light to be packed at the tip of the pipette as excitons passed through a region of the pipette that was below the cut-off frequency of the light. Experiments with this method indicated that, compared to an empty pipette without a crystal at the tip, one could get an amplification of two to ten times the intensity of light that could be passed through an empty pipette. In the present invention we describe a method that solves both the problems of feedback and of the intensity of the sub-wavelength light source.